Currently, Shipstation doesn't support Shopify subscription order handling for prepaid subscriptions. When a customer orders a prepaid subscription, Shopify treats it as one order, and updates that order every month to add a new item to be shipped. There's also an order queue.
When the order is fulfilled through Shipstation, it never comes back when a new item is added and Shopify changes from "fulfilled" to "unfulfilled". In one of the screenshots below, you can see the status is unfulfilled after three months of recurring orders (only one month fulfilled), with another 9 months planned out. But in the Shipstation window, it comes in as qty 12 (which is not the case).
